1. Field of the Invention
The present invention relates to a liquid crystal color display device and more particularly to a crystal color display device employing a liquid crystal element or unit as a light switching element.
2. Discussion of the Background
Heretofore, color display devices have typically employed CRTs (cathode-ray tubes) and have widely been used in television display units, OA (office automation) components, and other display units The CRT is a type of vacuum tube having a large conical shape. A display unit composed of a CRT must therefore be powered by a high-voltage power supply, requires a complex driver circuit, and is large in overall size, with the result that the display unit could not be thinned down beyond a certain limit.
Recent years have seen the development of panel-shaped liquid crystal display devices, which have resulted in pocketable liquid crystal color television receivers. As described in a Japanese periodical "Nikkei Electronics", 1984, 9-10, pages 211 to 240, such a pocketable liquid crystal color television receiver comprises a liquid crystal panel composed of a glass plate having red, green, and blue color filters assigned to each pixel, another transparent plate with an array of thin-film transistors disposed thereon, a TN (twisted nematic) liquid crystal material sealed between the plates, and a pair of polarizers sandwiching the plates and an illuminating light source positioned behind the liquid crystal panel Transparent pixel electrodes are arranged in alignment respectively with the pixel color filter elements. When the thin-film transistors are driven, signal voltages are applied to the pixel electrodes to control light transmission and shutoff.
Although the liquid crystal color display device described above can be constructed in a low profile, it is still disadvantageous in that color generation and reproducibility are insufficient due to the use of the optical filter, and in that displayed images have a low contrast and hence cannot be seen well.
With the prior liquid crystal color display device employing a glass plate or silicon subtrate, the pixel color filter elements must to be fabricated on the inner surface of the plate or substrate since if the color filter pattern were formed on the outer surface of the plate or substrate in corresponding relation to the transparent pixel electrodes on the inner surface of the plate or subtrate, color shifts would be generated due to the thickness of the plate or substrate. According to one method to achieve this an arrangement, the color filter elements are first formed on the inner surface of the plate or subtrate, and then an ITO film is formed on the color filters with the transparent pixel electrodes on the ITO film. According to another method, the transparent pixel electrodes are formed on the plate or substrate, and the color filter elements are formed on the transparent pixel electrodes. These methods have not been completely advantageous however. The color filter elements are formed by the dyeing process or the electrodepositing process. In order to provide better color reproducibility of the color display it is preferable to increase the thickness of the filter layer up to a range of from a few to ten micrometers. In case the color filter is formed directly on the inner surface of the plate or substrate, the increased thickness of the color filter fails to make the filter layer surface smooth when a transparent electrically conductive film is to be formed on the color filter. Where the color filter elements are to be formed on the transparent pixel electrodes, the threshold voltage to be applied when driving the liquid crystal is increased since the color filter itself is an insulating layer. As a consequence, the color filter is required to be of a reduced thickness which cannot however accomplish sufficient color reproduceability.